An optical frequency comb generating method of simultaneously generating plural optical frequency components having a frequency difference of an equal interval has attracted attention. This technique is used for a wavelength multiplexing light source of an optical wavelength division multiplexing communication system or a short-pulse light source for ultrafast transmission or optical measurement.
As disclosed in PTL 1, an optical frequency comb generating device has been proposed which employs a so-called dual-driven Mach-Zehnder type optical modulator that independently modulates optical waves propagating in two branch waveguides of a Mach-Zehnder type optical waveguide.
PTL 1 discloses a method of providing a flat power spectrum characteristic or a linear chirp characteristic using the optimal frequency distribution intensity of an optical frequency comb. It is described therein that it is necessary to satisfy a first condition expressed by Expression 1.Expression 1ΔA+Δθ=π  (1)
When the condition of Expression 1 is satisfied, the power spectrum is flat and the chirp is almost linear. Here, ΔA≡A1−A2 and Δθ≡θ1−θ2 are defined, A1 and A2 represent degrees of modulation in the branch waveguides, and θ1 and θ2 represent phase leads due to the optical path length or the bias control in the branch waveguides.
When a second condition of Expression 2 is satisfied, the comb signal conversion efficiency is maximum and the output/input ratio η in optical power is 0.5. Since Expression 2 is included in the condition of Expression 1, Expression 2 is the optimal drive condition.
      Expression    ⁢                  ⁢    2                                            Δ            ⁢                                                  ⁢            A                    =                      Δθ            =                          π              2                                                            (          2          )                    
A drive condition control method of causing the drive condition of an optical modulator to stably satisfy Expression 2 over a long period is disclosed in NPL 1. FIG. 1 is diagram schematically illustrating the control method disclosed in NPL 1.
An optical modulator 4 is provided with a Mach-Zehnder type optical waveguide 44, and two branch waveguides are provided with optical modulation parts 41 and 42 that can be independently driven, respectively. A phase regulator 43 is disposed as means for regulating a phase difference between optical waves propagating in the two branch waveguides.
An optical wave which is emitted from a continuum light source 1 such as a semiconductor laser light source and of which the polarization plane is adjusted by the use of a polarization controller 2 is input to the optical modulator 4. An RF signal supplied from an RF signal source 6 is divided into two RF signals by a distributor 7 and the divided RF signals are supplied to the modulation parts of the optical modulator. The signal intensity of one RF signal is adjusted through the use of an attenuator 8 and the adjusted RF signal is supplied to the optical modulator part.
In this method, four parameters of a degree of modulation A1 of a first drive signal, a degree of modulation A2 of a second drive signal, power Pin of a light beam input to the optical modulator, and power Pout of a light beam output from the optical modulator are monitored, and a DC bias to be supplied to the phase regulator 43 is controlled by the use of a bias control circuit 10 so as to satisfy Expressions 3 and 4. In FIG. 1, in order to monitor the ratio of the power Pin of an input light beam and the power Pout of an output light beam, some optical waves are extracted by the use of optical couplers 3 and 5 disposed in the waveguides and are input to a balanced light-receiving element 9. In order to monitor the degrees of modulation A1 and A2, a circuit (not shown) detecting the intensity of the RF signals supplied to the optical modulation parts is provided.
      Expression    ⁢                  ⁢    3                                                          Δ              ⁢                                                          ⁢              A                        =                                                            A                  1                                -                                  A                  2                                            =                              π                2                                              ⁢                                          ⁢                      Expression            ⁢                                                  ⁢            4                                                (          3          )                                              η          =                                                    P                out                                            P                in                                      =            0.5                                                (          4          )                    
In this way, in the method disclosed in NPL 1, it is necessary to monitor four parameters shown in Expressions 3 and 4 in order to control the drive condition. However, in order to know the four parameters, a part of two RF powers input to the MZ modulator, a part of optical power input to the MZ modulator, and a part of optical power output from the MZ modulator are actually monitored as shown in FIG. 1. Accordingly, it is necessary to previously obtain the relationship between the actually-monitored parameters and four parameters corresponding thereto in a one-to-one manner, that is, calibration is necessary.
The relationship between the power and the degrees of modulation (A1 and A2) of RF signals to be monitored can be very accurately obtained at a relatively low cost and does not present any particular difficulty. However, since there is an excessive loss due to the internal structure of the optical modulator and a coupling structure associated with input and output of a light beam, it is difficult to accurately calculate the input optical power (Pin) and the output optical power (Pout).
For example, PTL 2 discloses a method of obtaining a degree of modulation by measuring plural sideband peak intensity using a light spectrum analyzer. On each side of dual drive, the actual drive condition (modulation frequency and power) can be calibrated by obtaining the degree of modulation and the monitored RF power at a certain point. Since the degree of modulation can be directly measured using only the light spectrum analyzer without changing the wiring and configuration of the device, it is possible to very accurately obtain the relationship between the RF power and the degree of modulation.
The power ratio η=0.5 of the input and output light beams to be controlled is not a singular point. Accordingly, when there is an error in the calibration performed for the first time, the error is not ascertained and remains as an error, thereby causing degradation in signal quality.
In another calibration method, the optimal drive condition can be obtained from the temporal waveform of an output light beam and the power ratio of monitored light beams associated with the input and output at that time can be controlled to a target control value. However, when it is driven at ultrahigh frequencies of 10 GHz or higher, there is a problem in that a high-cost measuring instrument is necessary for calibration. When the calibration can be accurately performed, but an excessive loss in the modulator varies over a long period and thus the optimal control point varies, the related art has a problem in that the variation of the optimal control point is not recognized in the control method and thus the degradation of signals is not recognized.